Electric furnace and the method of operating the same



y 9, 1932- R. c. BENNER ET AL ELECTRIC FURNACE AND THE METHOD OF OPERATING THE SAME Filed Oct. 2, 1950 2 sheets-sheet 1 INVENTORS RAYMOND 2. BENNER GEORGE J. EASTER BY CLARENCE E.HAWKE.

July 19, 1932. R. c. BENNER ET AL 1,867,646

ELECTRIC FURNACE AND THE METHOD OF OPERATING THE SAME Filed Oct, 2, 1930 2. Sheets-Sheet Pay. 8.

k v X n INVENTORS RAYMOND c. szunzw. GEORGE JV EASTER BY CLARENCE E.HA\NKE.

mm mac/C ATTORNEY Patented July 19, 1932 warren stares RAYMOND C. IBENNER AND GEORGE J. EAST CLARENCE E. HA'WKE, F METUCHEN,

NIAGARA FALLS, NEW YORK, A CORPORATIQN 0F PENN- ;eoaunnn'm COMPANY, or srnvanra ER, 015 NIAGARA FALLS, NEW YORK, AND

NEW JERSEY, ASSIGNORS TO THE CAR- ELECTBIU FURNACE AND THE METHOD OF @PERATING THE SAME Application filed October 2, 1930, Serial No. 485,998, and in This invention relates to the operation of non-metallicresistors, and especially to the operation of resistors adapted for use at temperatures above 1,100 G, where metallic resisters made from the usual commercial materials cannot be employed. The present ap plication is a continuation in part of our c0- pending application, United States Serial No. 324,419 filed December 7, 1928. This c0- pending application discloses the use of nonmetallic resistors having a negative temperature coefiicient of resistance in series with non-metallic resistors having a positive temperature coeflicient, in order to maintain a uniform power load and hence a uniform temperature within an electric furnace.

Although the attainment of high temperatures is made possible by the use of non-metallic resistors containing carbon as a principal constituent, the operation of these resistors in an electric furnace often involves certain difiiculties. In general, the electrical resistance of a non-metallic heating element does not remain constant with changes in temperature, but varies over a considerable range. The variation in resistance is often appreciable for even small changes in temperature, and the fluctuations in current caused by the change in resistance make difiicult the control of both temperature and the power input of the furnace.

If the resistance of the heating element increases with rising temperature, the power input of the furnace at constant voltage will diminish as the furnace becomes heated to too high a temperature, and in many cases this decreased power input with rising temperature is desirable as a means of regulation; however, if the electrical resistance of the element decreases with rising tempera ture, the condition is reversed, and as the furnace becomes heated the power input, instead of being diminished, is still further increased. The danger of such a situation, when delicate temperature control is required, is evident.

l/Vith most non-metallic resistors the temperature coefficient of resistance is negative, the electrical resistance decreasing considerably with rising temperature. The actual magnitude of the effect depends upon the type Canada March 20, 1929.

of resistor used, and may vary over a wide range even with resistors made from practically the same material. Diihculties resulting from a negative resistance-temperature coeflicient are encountered with resistors of carbon, graphite, and particularly with elements composed of silicon carbide. Silicon carbide heating elements have been observed in which the temperature coefficient of resistance between 1,000 and 1,500 C. was so strongly negative that the resistors disintegrated with almost explosive violence when allowed to operate within the above temperature range unless the applied voltage was continuously reduced as the resistor temper ature increased. The disintegration of the resistor is caused by accumulative'interactions in which the increase in temperature causes an increase in current, and the increased current-in turn causes a still further increase in temperature, until the resistor is .destroyed by "the excessive temperature developed.

Previous efforts to overcome the difficulties resulting from the change in resistance with temperature have been largely confined to regulation by means of ballast resistances or 1 other electrical means located externally with respect to the furnace. For a ballast resistance to be of value in preventing the resistor from overload with increasing temperature, the resistance must be of the same order of magnitude as that of the resistor itself. For this reason the power loss in the external resistance maybe almost as high as the power utilized in the furnace. Such a mechanism is thus both inefficient and undesirable. The regulation of the applied voltage by means of a variable volt-age transformer is more satisfactory, but with elements having a pro nounced negative temperature characteristic at the temperature of operation, such a meansvof regulation requires the constant attention of an operator.

In our improved method of operation, we are able to utilize resistors having a negative temperature coefficient of resistance throughout the entire range of operating temperatures by connecting them in series with nonmetallic resistors in which the negative resistance-temperature characteristic at the temperature of operation is lacking. in addi tion, we are able to provide a non-metallic resistance unit in which the resistance is practically independent of temperature. lhe elements acting as a controlling resistance are placed within the furnace, and, although they afford a degree of current regulation comparable with that produced by external regulating means, the power is entirely converted to useful heat. As a controlling resistance for elements having a negative temperature coeficient, we prefer to employ resistors having a positive temperature coetficient throughout the greater portion of the operating temperature range.

Although a negative temperature coenicient of resistance is characteristic of most non-metallic resistors, there are certain types of non-metallic elements in which the temperature coefiicient is either zero or positive through a restricted range. ln the case of silicon carbide resistors the temperature coeificient depends to a large extent upon the method of manufacture. Resistors are available, as will he more fully explained below,

which vary from one having a negative (20- eflicient throughout the entire range of temperatures, to one in which the resistance actually increases with rising temperature between 700 and 1,500 C.

In carrying out our invention, any type of electric furnace may be employed, providing two or more resistors can be used. The element having a negative temperature coeiiicient in the operating range may consist of graphite, carbon, orv silicon carbide. The drawings illustrate'specific examples of the resistance temperature characteristics of elements that may be used, and also show as an example a furnace which has been found especiall satisfactory for carrying out our metho of operation in the case of silicon carbide resistors.

In the drawings:

Figure 1 shows the variation in resistance with temperature in the case of silicon carbide resistors;

Fi re 2 is a vertical section oi a furnace in w 'ch the resistor rods pass through the heatin chamber, the section being taken throng one of the resistor rods, and

Figure 3 is a vertical section taken along the line 33 oi Figure 1.

In Figure 1, curve A shows the variation in electrical resistance with temperature of a silicon carbide heatingelement in which the temperature coeficient of resistance is negative throughout the entire temperature range. It will he observed that the resistance at l,500 C. is less than. half that which olotains at 900 (3., so that in passing through this temperature range the current con stant voltage would he more than doubled.

Curve shows the variation in resistance znseaeae with temperature of a silicon carbide re sister in which the temperature coefficient of resistance is positive between '8' 50 C. and 1,500 O, the increase in resistance being approximately 30 per cent.

Any negative value for the temperature coefficient of resistance may be considered as being less than zero. I By the expression at least zero we mean any coeilicient which has a value of zero or greater in a positive direction.

Ely connecting in series the elements having the resistance-temperature characteristics described above, the increase in current with rising temperature when the resistor is operatedat constant voltage can be greatly decreased. The following table is calculated rrom the two curves shown in Figure l:

Resistor Resistor A-l-B 1n A. B series Resistance at- Ohms Ohms Ohms Amps Amps Amps.

element when the voltage is constant for a i.

given resistor (or pair of resistors in series). The following values are calculated from the above table to show the increase in power consumption at l,500 C. in comparison with that consumed at 900 (3.:

A+B in Resistor A SBIIGS Increase in power consumption 10 Thus, if a resistor-having the characteristics shown in curve A is employed alone, the power at constant voltage will be more than doubled in passing through the temperature 0., whereas if the range of 900 G. to 1,500

two resistors are connected in series, the 1ncrease 1n power input in passmg through the same range of temperature is only 12 per cent. This small increase in power over a elnninate temperature range of 600 C. will the necessity oi voltage regulation to correspond with small variations in temperature, and at the some time will prevent the re-.

sister from running away or destroying it- M which results when self from the overload the resistance decreases rapidly with temperature. The great increase of radiation at the higher temperature ranges will tend to prevent too great an increase of tempera- I '"ture.

memes When a resistor having the characteristics shown in curve A in Figure 1 is connected in series with a resistor having a zero temperature coefiicient through the operating range, the control afforded is equal to that obtained by using an external ballast resistance, although the efiect is not as pronounced as that shown in the preceding tables. Assuming a constant resistance of 5.4 ohms for the resistor B, the increase in power consumption between 900 and 1,500 will be decreased from 108% (for the resistor A used alone) to approximately per cent for the two resistors connected in series. If the element having a constant resistance-temperature characteristic is of somewhat higher resistance than the element having a negative temperature coefficient, the increase in power consumption will be considerably less than 40 per cent.

The furnace shown in Figure 2 is especially adapted for maintaining a uniform tempera ture and is described and claimed in our copending application, United States Serial No. 324,419 filed December 7 1928. The use of the method of operating non-metallic reslstors described in the present application ofiers particular advantages in connection with this type of furnace, The resistors 1 are placed directly within the heating chamber, and are therefore subject to material temperature fluctuations when the charge is introduced Under these conditions it is often particularly advantageous that the electrical resistance be practically in dependent of the temperature as, if the resistance decreases with increasing temperature, the furnace will receive heat at a decreased rate when the charge is first 1nserted (making it slower heating up) and tends, as previously explained, to run away by overheating at a later stage in the operation. Pairs of resistors, denoted in Figure 3- by 1A and 1B one member of each pair having a negative temperature coeflicient through the operating range and the other functioning as a control or ballast resistance as previously described, are connected electrically so that the resistors of each pair will be inseries. The lining 3 of the heating chamber is preferably constructed of refractory having high thermal conductivity in order to attain a uniform heat distribution and to supply a heat reservoir to take care of fluctuations in temperature during the loading and unloading of the furnace, which, in the usual method of operation, is charged intermittently without being allowed to cool. The inner lining 3 of the combustion chamber should have a thermal conductivity greater than .006 cal./cm /sec./C., and may be composed, for example, of silicon carbide or fused alumina. The inner lining is backed up by suitable insulating material 5. The spaces 4 between the inner and outer lining cut down losses by tion of grain of the ture coefficient may conduction of heat. The resistors are supported by and electrically connected through the water cooled terminals 2, the detailed structure of which is described 7 in United States Patent No. 1,742,286, issued to Harold N. Shaw, January 7, 1930. I

A simple test for determining the resistance temperature characteristics of a resistor in order to ascertain its suitability for use in our method of operation, consists in connecting the resistor in series with an ammeter and applying a voltage somewhat in excess of that necessary to produce the normal operating temperature of the element. If the temperature coefiicient of resistance is negative throughout the operating range, the current as indicated by the ammeter will increase continually until the maximum temperature is attained, but if the coeificient is positive in the range of operation the current will first increase to a maximum value and then decrease as higher temperatures are attained.

Resistors as made from ordinary silicon carbide bonded by recrystallizing under the influence of heat have a strongly negative temperature-resistance coeflicient over the entire temperature range as shown in curve A. If, however, the silicon carbide be treated first with sulfuric acid and later with a caustic solution and then carefully washed, it is found that the resistors produced therefrom have a positive coeflicient over part of the range as in curve B. By varying the proportwo types the temperabe made to have any de-' slred value between the other two. The meth, 0d of controlling the positive temperature coeflicient of resistance of a silicon carbide resistor by chemical treatment of the grain is described and claimed in a co-pending application of John A. Boyer and Almer J. Thompson, Serial No. 501,020, filed Decemher 9, 1930.

We claim:

The method of electrical heatin which comprises connecting a silicon carbide heating element having a negative temperature coefiicient of electrical resistance throughout the principal operating range in series with a slliconcarbide heating element having a positive temperature coeflicient of electrical resistance within the operating range and generating heat within an electric furnace from both of the said heating elements.

2. In an electric furnace a heating chamber, and a plurality of silicon carbide electrical resistors for heating said chamber, one or more of said resistors having a negative temperature coefficient of electrical resistance and the remainder having a positive temperature coefficient of electrical resistance in the principal operating range, each of the resistors possessing a negative temperature coefiicient of electrical resistance being connected in series with a resistor having a positive temperature coefficient of electrical resistance;

3. In an electric furnace the combination comprising a heating chamber, a plurality of silicon carbide resistors extending through said chamber, one or more of said resistors having a negative temperature coefficient of electrical resistance and the remainder a positive temperature coeflicient of electrical resistance in the principal operating range, each of the resistors of negative coeflicient of electrical resistance being connected in series with a resistor of positive coefiicient of electrical resistance, and a lining for said heating chamber composed principally of silicon carbide.

4. The method of operating a non-metallic resistor having a negative temperature coeflicient of electrical resistance throughout the range of temperatures in which the resistor is operated, which comprises connecting the said resistor in series with a silicon carbide resistor having a positive temperature coefficient of electrical resistance at a temperature at which the said silicon carbide resistor is operated, supplying current to the connected resistors at approximately constant voltage, and generating heat within an electric furnace from both of the said resistors.

5. The method of utilizing the heat from a ballast resistance connected in series with a non-metallic resistor for the purpose of counteracting the negative temperature coefficient of electrical resistance of the said resistor, which comprises forming the said ballast resistance from a silicon carbide body having a positive temperature coeflicient of electrical resistance, and operating the said ballast resistance within an electric furnace.

6. The method described in claim 5 in which the ballast resistance is operated at a temperature above 1100 C.

7. Themethod of operating a non-metallic resistor having a negative temperature coefficient of electrical resistance at the tempera-' ture of operation, which comprises connecting the said resistor in series with a silicon carbide resistor having a temperature coefficient of at least zero at the temperature at which the said silicon carbide resistor is op erated, and operating both resistors within an electric furnace.

In testimony whereof we afiix our signatures.

i RAYMOND C. BENNER.

GEORGE J. EASTER. CLARENCE E. HAVVKE. 

